The qualitative aspect has to do with ensuring that the space has a pleasing feel and ambiance. It is the artistic interspersing of light and shadow, of illumination and darkness, of figure and form.

The quantitative aspect revolves around making sure that there is adequate light for the task at hand. The Illuminating Engineering Society (IES) of North America publishes guidelines of light levels for many tasks and activities based on the nature of the task, the size of objects handled, the detail required, the average age of the people in that space and so on. A typical office is lit to an illumination of 30 to 100 "footcandles". Light levels can also be expressed in the metric unit "lux"; 1 footcandle is approximately 10 lux.

The rate of energy consumption is called "power" and is measured in watts. A 200-watt lamp is consuming energy at twice the rate of a 100-watt lamp. The electric company charges consumers for the total amount of energy they consume. This is measured in kilowatt-hours or kwh. A 200-watt lamp burning for 5 hours consumes 1,000 watt-hours of energy, which is called a kilowatt-hour. Note that burning a thousand watt lamp for one hour costs the same as burning a hundred watt lamp for ten hours.

On the average, electricity costs about 10 cents per kilowatt-hour. It is as high as 20 cents per kwh in some places and as low as 5 cents in others.

A 100-watt lamp burning for 10 hours is 1 kwh, which costs 10 cents (average), that is, it costs a penny an hour. Electric lighting is one of the best values you can get for your penny!